What screening to protect mic from cellphone interference?

What screening is necessary if I want to protect a domestic-grade
electret microphone from the RF coming from a cellphone which is in use?

I sometimes hear radio or TV news broadcasts which still get cellphone
interference so I would guess it is not all that easy to provide a
reasonale level of screening.

On the other hand, you can get a cheap electret microphone designed to
be worn in the ear (as a phone-recording microphone) which is remarkably
resistent to cellphone RF.
http://www.teknikmagasinet.se/prod/s...png/290080.png

How do I make a domestic electret microphone capsule with its co-ax
microphone cable (cdonsisting of shield and one core) resistent to
cellphone RF?


Moz

--



Note: xposted to 4 relevant groups (GKSA limit) = alt.cellular
alt.engineering.electrical sci.electronics.design uk.telecom.mobile

? <cs_posting@hotmail.com> ?????? ??? ??????
news:1172031439.228710.31640@l53g2000cwa.googlegro ups.com...
> On Feb 20, 7:21 pm, Mozzy <d...@account.com> wrote:
>
> > On the other hand, you can get a cheap electret microphone designed to
> > be worn in the ear (as a phone-recording microphone) which is remarkably
> > resistent to cellphone RF.
>
> Generally the way intereference gets in is by speaker or power or
> signal wires acting as antennas. If a wire doesn't need to carry a
> high speed signal, you can bypass it with an RF choke in series and/or
> capacitors to ground. And put the circuit in a shielded enclosure.
> Chokes, and perhaps coaxial microphone cable, should help quite a bit.
>
> With GSM phones, the worst interference seems to be when the network
> is getting ready to ring them. In fact, I can often tell when my
> phone is about to ring because of what happens to my computer
> speakers, or car radio.
Err...I hope that it doesn't do something similar to our brains?(Just now
reading Stephen King's cell).I have measured the electrostatic field of my
Nokia 1100, and is 11 V/m at, maybe, 1/2", when calling (when idle,
something like 0.44 V/m)




> Chances are the phone won't be heard in its
> own headphone because it probably has it's audio jack bypassed with
> chokes and its amplifier not only well shielded, but also muted at
> this point. And nobody is yet on the other end to hear if the
> microphone is being interfered with. Presumably a phone that does
> voice recognition could even know exactly when it is transmitting its
> stronger pulsed signals, and thus if necesssary not listen right then,
> to avoid confusing itself.
>



--
Tzortzakakis Dimitrios
major in electrical engineering
mechanized infantry reservist
dimtzort AT otenet DOT gr

In message <Xns98DE3A21981701A4D@127.0.0.1>
Mozzy <demo@account.com> wrote:

> What screening is necessary if I want to protect a domestic-grade
> electret microphone from the RF coming from a cellphone which is in use?
>
> I sometimes hear radio or TV news broadcasts which still get cellphone
> interference so I would guess it is not all that easy to provide a
> reasonale level of screening.
>
> On the other hand, you can get a cheap electret microphone designed to
> be worn in the ear (as a phone-recording microphone) which is remarkably
> resistent to cellphone RF.
> http://www.teknikmagasinet.se/prod/s...png/290080.png
>
> How do I make a domestic electret microphone capsule with its co-ax
> microphone cable (cdonsisting of shield and one core) resistent to
> cellphone RF?

It is exceptionally unlikely to be the microphone that's causing
the problem; it will be the electronics that it's connected to
that is demodulating the signal, and therefore that needs to be
filtered and/or screened.

Unless you understand electronics fairly well, it's difficult or
impossible to explain how to do this. If you /do/ understand
electronics, then the best hint is to beware of the inductances
of even short lengths of wire. Surface mounted components and
tiny track lengths are your friends.

Dave